Update coding style to include PSR-12, among other options

This also includes heavy use of slevomat/coding-standard to apply
various checks to the code, based on maintainer (me) preference.

tests/Codec/GuidStringCodecTest.php

@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Ramsey\Uuid\Test\Codec;
 
 use PHPUnit\Framework\MockObject\MockObject;
@@ -35,7 +37,8 @@ class GuidStringCodecTest extends TestCase
             'time_hi_and_version' => 'abcd',
             'clock_seq_hi_and_reserved' => 'ab',
             'clock_seq_low' => 'ef',
-            'node' => '1234abcd4321'];
+            'node' => '1234abcd4321',
+        ];
     }
 
     protected function tearDown(): void
@@ -73,7 +76,6 @@ class GuidStringCodecTest extends TestCase
         $this->assertEquals('12345678-1234-abcd-abef-1234abcd4321', $result);
     }
 
-
     public function testEncodeBinaryUsesFieldsArray(): void
     {
         $this->uuid->expects($this->once())